Optical systems having a real image focus can receive collimated light and focus it at a point. Such optical systems can be found in nature, e.g., human and animal eyes, or can be man-made, e.g., laboratory systems, guidance systems, etc. In either case, aberrations in the optical system can affect the system's performance. By way of example, the human eye will be used to explain this problem.
Referring to FIG. 1A, a perfect or ideal eye 100 is shown diffusely reflecting an impinging light beam (not shown for sake of clarity) from the back of its retina 102 (i.e., the fovea centralis 103) through the eye's optics to include lens 104 and cornea 106. For such an ideal eye in a relaxed state, i.e., not accommodating to provide near-field focus, the reflected light (represented by arrows 108) exits eye 100 as a sequence as of plane waves, one of which is represented by straight line 110. However, an eye normally has aberrations that cause deformation or distortion of the wave exiting the eye. This is shown by way of example in FIG. 1B where aberrated eye 120 diffusely reflects an impinging light beam (again not shown for sake of clarity) from the back of its retina 122 of the fovea centralis 123 through lens 124 and cornea 126. For aberrated eye 120, reflected light 128 exits eye 120 as a sequence of distorted wavefronts, one of which is represented by wavy line 130.
Currently, there are a number of technologies that attempt to provide the patient with improved visual acuity. Examples of such technologies include remodeling of cornea 126 using refractive laser surgery or intra-corneal implants, and adding synthetic lenses to the optical system using intraocular lens implants or precision-ground spectacles. In each case, the amount of corrective treatment is typically determined by placing spherical and/or cylindrical lenses of known refractive power at the spectacle plane (approximately 1.0-1.5 centimeters anterior to cornea 126) and asking the patient which lens or lens combination provides the clearest vision. This is obviously a very imprecise measurement of the true distortions in wavefront 130 because 1) a single spherocylindrical compensation is applied across the entire wavefront, 2) vision is tested at discrete intervals (i.e., diopter units) of refractive correction, and 3) subjective determination by the patient is required in order to determine the optical correction. Thus, the conventional methodology for determining refractive errors in the eye is substantially less accurate than the techniques now available for correcting the ocular aberrations.
One method of measuring ocular refractive errors is disclosed by Penney et al. in "Spatially Resolved Objective Autorefractometer," U.S. Pat. No. 5,258,791, issued Nov. 2, 1993. Penney et al. teach the use of an autorefractometer to measure the refraction of the eye at numerous discrete locations across the corneal surface. The autorefractometer is designed to deliver a narrow beam of optical radiation to the surface of the eye, and to determine where that beam strikes the retina using a retinal imaging system. Both the angle of the beam's propagation direction with respect to the optical axis of the system and the approximate location at which the beam strikes the corneal surface of the eye are independently adjustable. A small uncertainty or error in the location of the beam's point of incidence on the cornea exists due to the curved corneal surface. For each point of incidence across the corneal surface, the refraction of the eye corresponding to that surface point can be determined by adjusting the angle at which the beam strikes the cornea until the beam refracted on to the iris strikes the fovea centralis. Adjustment of the beam angle of propagation can be accomplished either manually by the patient or automatically by the autorefractometer if a feedback loop involving a retinal imaging component is incorporated.
Penney et al. further teach the use of the autorefractometer measurements in determining the appropriate corneal surface reshaping to provide emmetropia. This is accomplished by first obtaining accurate measurement of corneal surface topography (using a separate commercially available device). A mathematical analysis is then performed using the initial corneal topography at each surface reference point, the measured refraction at each surface point, and Snell's law of refraction, to determine the required change in surface contour at each reference point. The contour changes at the various reference points are then combined to arrive at a single reshaping profile to be applied across the full corneal surface.
The major limitation to the approach described by Penney et al. is that a separate measurement of corneal topography is required to perform the Snell's Law analysis of needed refraction change. This requirement adds significantly to the time and cost of the complete diagnostic evaluation. Furthermore, the accuracy of the refraction change analysis will be dependent on the accuracy of the topographic measurement and the accuracy of the autorefractometer measurement. In addition, any error in the spatial orientation of the topography "map" with respect to the refraction map will degrade the accuracy of the needed correction profile.
A second limitation to the approach described by Penney et al. is that test points on the corneal surface are examined sequentially. Eye motion during the examination, either voluntary or involuntary, could introduce substantial errors in the refraction measurement. Penney et al. attempt to provide detection of such eye movement by deliberately including measurement points outside the pupil, i.e., in the corneal region overlying the iris, where the return from the retina will obviously be zero at specific intervals in the examination sequence. However, this approach may still allow substantial undetected eye movement error between such iris reference points.
At present, no corrective method is based on the concurrent examination of the complete distortions in wavefront 130. Measurement of wave aberrations of the human eye, i.e., ocular aberrations, has been studied for a number of years. One prior art method and system are disclosed by Liang et al. in "Objective Measurement of Wave Aberrations of the Human Eye With the Use of a Hartmann-Shack Wave-front Sensor," Journal of the Optical Society of America, Volume 11, No. 7, July 1994, p.p. 1949-1957. Liang et al. teach the use of a Hartmann-Shack wavefront sensor to measure ocular aberrations by measuring the wavefront emerging from the eye by the retinal reflection of a focused laser light spot on the retina's fovea. The actual wavefront is reconstructed using wavefront estimation with Zernike polynomials.
The Hartmann-Shack wavefront sensor disclosed by Liang et al. includes two identical layers of cylindrical lenses with the layers arranged so that the lenses in each layer are perpendicular to one another. In this way, the two layers act like a two-dimensional array of spherical lenslets that divide the incoming light wave into subapertures. The light through each subaperture is brought to focus in the focal plane of the lens array where a charge coupled device (CCD) image module resides.
The system of Liang et al. is calibrated by impinging an ideal plane wave of light on the lenslet array so that a reference or calibrating pattern of focus spots is imaged on the CCD. Since the ideal wavefront is planar, each spot related to the ideal wavefront is located on the optical axis of the corresponding lenslet. When a distorted wavefront passes through the lenslet array, the image spots on the CCD are shifted with respect to the reference pattern generated by the ideal wavefront. Each shift is proportional to the local slopes, i.e., partial derivatives, of the distorted wavefront which can be used to reconstruct the distorted wavefront, by means of modal wavefront estimation with Zernike polynomials.
However, the system disclosed by Liang et al. is effective only for eyes having fairly good vision. Eyes that exhibit considerable myopia (near-sightedness) would cause the focus spots to overlap on the CCD thereby making local slope determination impossible for eyes having this condition. Similarly, eyes that exhibit considerable hyperopia (farsightedness) deflect the focus spots such that they do not impinge on the CCD thereby again making local slope determination impossible for eyes having this condition.
Another limitation of the system of Liang et al. is the configuration of the Hartmann-Shack sensor in that the lenses must be uniform in order to define a uniform lenslet array so that the entire array shares a common focal plane and does not itself induce distortions in the wavefront. However, the manufacturing costs associated with such constraints are considerable.
Thus, owing to all of the above-noted limitations, Liang et al. can only achieve wavefront measurement for a relatively small class of patients. Such patients can have, at most, mildly distorted vision.